We are developing a model to characterize the spatial-frequency dependence of detective quantum efficiency, DQE(f). The transfer of energy through a fluorescent screen is modelled as a cascade of stochastic processes. We have improved on existing models in three ways: by including the effect of finite phosphor thickness, by including the possibility of secondary quantum sinks especially at high spatial frequencies, and by using poly-energetic x-ray spectra. This paper deals mainly with the effect of the finite phosphor thickness on signal and noise, i.e., the difference in transfer of signal and noise by the phosphor. It has been observed that for film-screen systems the shape of the noise power spectrum (NPS) does not match the give of the modulation transfer function (MTF) squared. Even after correction for film granularity, MTF falls off faster than NPS. This causes a significant reduction in the DQE at high spatial frequencies. We have found that our model can predict MTF and the shape of the quantum noise power spectrum for an ortho-M/Min-R film-screen system. Our model confirms the hypothesis that the finite phosphor thickness results in the noise being transferred more efficiently than the signal at high spatial frequencies. This is in part responsible for the decline in DQE(f) with increasing spatial frequency.